It is known to produced rows of cylindrical lenses, e.g., by grinding and polishing with a concave cylindrical rod and then turning the raster through 90° to produce cross-cylindrical lenses, which may be different from true axially symmetrical lenses in that they will be well below the diffraction limit for such extremely weak lenses, e.g., with 1 mm square aperture and 80 mm focal length. (http://www.eso.org/gen-fac/pubs/messenger/archive/no.114-dec03/mess-wilson. pdf)
JSW Corporation is known to offer an apparatus for performing crystallization of amorphous silicon thin films for the purpose of recrystallization of the amorphous silicon thin film into poly-crystalline silicon (“Poly-si” or “poly”) for the purpose of making on a substrate, e.g., thin film transistors, e.g., for flat panel displays. This is known as excimer laser annealing “ELA”) due to the utilization of an excimer, e.g., XeCl laser for the light energy source to do the ELA annealing with the resultant melting and recrystallization of the amorphous silicon to form the thin film poly layer, wherein, e.g., the thin film transistor gates are formed.
JSW has demonstrated a capability of working with substrates with, e.g., 370×470 mm surface areas and 730×920 surface areas, with the former being worked with a tool having a beam size of 365×0.4 mm, an excimer laser repetition rate of 300, an overlap of 95%, a scan pitch of 0.02, a scan length of 470, a number of scan equal to 1, performing a total of 23500 crystallization shots per sheet, with a crystallization time of 78 seconds per sheet utilizing 21390 wasting shots per screen, resulting in 44,890 total shots per screen and a 150 second time of actuation per screen for a throughput of 24.06 screens per hour, with similar numbers in the later case resulting in 2 scans to cover the sheet in the second case with the wider (920 mm) sheet, a 157 second per sheet crystallization time, 16698 wasting shots per sheet 229 seconds time of actuation and a 15.70 sheets per hour throughput. Lambda Physik offers a machine with roughly similar capabilities.
H. Kahlert, et al., “High-resolution optics for thin Si-film crystallization using excimer lasers: present status and future development,” Proc. or SPIE-IS&T, Electronic Imaging, SPIE Vol. 5004 (2003), pp. 20-27 (“Kahlert”) discusses forming a several hundres μm wide by 370 mm long beam at the workpiece for super-lateral-growth (“SLG”) crystallization. Also discussed is sequential lateral solidificationn as is also discussed in a number of patents issued to Im, including U.S. Pat. Nos. 6,322,625, issued on Nov. 27, 2001, entitled CRYSTALLIZATION PROCESSING OF SEMICONDUCTOR FILM REGIONS ON A SUBSTRATE, AND DEVICES MADE THEREWITH, based on U.S. application Ser. No. 09/200,533, filed on Nov. 27, 1998; and U.S. Pat. No. 6,368,945, issued on Apr. 9, 2002, entitled METHOD AND SYSTEM FOR PROVIDING A CONTINUOUS MOTION SEQUENTIAL LATERAL SOLIDIFICATION, based on U.S. application Ser. No. 09/526,585, filed on Mar. 16, 2000; and U.S. Pat. No. 6,555,449, issued to Im et al. on Apr. 29, 2003, entitled METHODS FOR PRODUCING UNIFORM LARGE-GRAINED AND GRAIN BOUNDARY LOCATION MANIPULATED POLYCRYSTALLINE THIN FILM SEMICONDUCTORS USING SEQUENTIAL LATERAL SOLIDFICATION, based on U.S. application Ser. No. 09/390,535, filed on Sep. 3, 1999; and U.S. Pat. No. 6,563,077, issued on May 13, 2003, entitled SYSTEM FOR PROVIDING A CONTINUOUS MOTION SEQUENTIAL LATERAL SOLIDIFICATION, based on U.S. application Ser. No. 09/823,547, filed on Mar. 30, 2001; and U.S. Pat. No. 6,573,531, issued to Im, et al. on Jun. 3, 2003, entitled SYSTEMS AND METHODS USING SEQUENTIAL LATERAL SOLIDIFICATION FOR PRODUCING SINGLE OR POLYCRYSTALLINE SILICON THIN FILMS AT LOW TEMPERATURES, based on U.S. application Ser. No. 09/390,537, filed on Sep. 3, 1999; and U.S. Pat. No. 6,582,827, issued on Jun. 24, 2003, entitled SPECIALIZED SUBSTRATES FOR USE IN SEQUENTIAL LATERAL SOLIDIFICATION PROCESSING, based on U.S. application Ser. No. 09/722,778, filed on Nov. 27, 2000, and also discussed in U.S. Published patent application Ser. No. 2003/0,096,489A1, with inventors Im, et al., published on May 22, 2003, entitled METHODS FOR PRODUCING UNIFORM LARGE-GRAINED AND GRAIN BOUNDARY LOCATION MANIPULATED POLYCRYSTALLINE THIN FILM SEMICONDUCTORS USING SEQUENTIAL LATERAL SOLIDIFICATION, based on U.S. application Ser. No. 10/294,001 , filed on Nov. 13, 2002; and U.S. application Ser. No. 2003/0119286A1, with inventors Im, et al., published on Jun. 26, 2003, entitled METHOD FOR PRODUCING UNIFORM LARGE-GRAINED AND GRAIN BOUNDARY LOCATION MANIPULATED POLYCRYSTALLINE THIN FILM SEMICONDUCTORS USING SEQUENTIAL LATERAL SOLIDIFICATION, based on U.S. application Ser. No. 10/308,958 , filed on Dec. 3, 2002.
Additional patents discuss aspects of such thin film crystallization, including, U.S. Pat. No. 5,432,122, issued to Chae on Jul. 11, 1995, entitled METHOD OF MAKING A THIN FILM TRANSISTOR BY OVERLAPPING ANNEALING USING LASERS, based on an U.S. application Ser. No. 147,635, filed on Nov. 3, 1993; and U.S. Pat. No. 6,177,301, issued to Jung on Jan. 23, 2001, entitled METHOD OF FABRICATING THIN FILM TRANSISTORS FOR A LIQUID CRYSTAL DISPLAY, based on an U.S. application Ser. No. 09/311,702, filed on May 13, 1999; and U.S. Pat. No. 6,316,338, issued to Jung on Nov. 13, 2001, entitled LASER ANNEALING METHOD, based on an application Ser. No. 09/605,409, filed on Jun. 28, 2000; and U.S. Pat. No. 6,215,595, issued to Yamazaki et al. on Apr. 10, 2001, entitled APPARATUS AND METHOD FOR LASER RADIATION, based on an U.S. application Ser. No. 09/583,450, filed on May 30, 2000; and U.S. Pat. No. 6,300,176, issued to Zhang et al. on Oct. 9, 2001, entitled LASER PROCESSING METHOD, based on an U.S. application Ser. No. 504,087, filed on Jul. 19, 1995, and U.S. Pat. No. 6,396,560, issued to Nogucbi et al. on May 28, 2002, entitled METHOD OF PRODUCING LIQUID CRYSTAL DISPLAY PANEL, based on an U.S. application Ser. No. 09/667,758, filed on Sep. 21, 2000, and U.S. Published patent application Ser. No. 2004/0060504A1, with inventors Takeda et al. , published on Apr. 1, 2004, entitled SEMICONDUCTOR THIN FILM AND PROCESS FOR PRODUCTION THEREOF, based on U.S. application Ser. No. 10/462,792, filed on Jun. 17, 2003.
Kalhert notes that the prior art is limited to a laser pulse dimension, line-width, at the workpiece of 400 μm in the prior art.
FIG. 1 shows an optical layout discussed in Lahlert for a Lambda Physik ELA machine. Turning to FIG. 1 it can be seen that the assembly 20. The assembly 20 comprises a laser output beam 22, e.g., the output of a XeCl LS 1000 excimer laser made by Lambda Physik. The laser beam 22 is passed through an attenuator 24, which comprises a pair of attenuator plates 26. The beam 22 is then expanded in the long axis by a long axis expansion optic 30, including a first telescope lens 32 and a second telescope lens 34, together forming an expanding telescopic element in the long axis of the beam 22. The beam is then passed through a long axis homogenizer 40 consisting of a first row of cylindrical lenses 42 and a second parallel row of cylindrical lenses 44, the cylindrical lenses of the first row 42 each having a focal point at a selected distance there between in the long axis, followed by an imaging lens 46, imaging the long axis of the beam. The beam is then passed through a short axis homogenizer 50, including a first row of cylindrical lenses 52 and a second row of cylindrical lenses 54, with each lens of the first row 52 having a focal point at a selected distance there between, and followed by an imaging lens 56, imaging the beam in the short axis at a slit 62 formed in a field stop 60, also including a field lens 64. The widened beam 80 is then rotated 90° by a mirror 90, having, a 90% reflectivity and then passed through a beam short axis magnifier 100, consisting of a 5× magnification cylindrical doublet having a first lens 102 and a second lend 104, which forms the final ELA beam 120 on or about the substrate 130. The prior art assembly 20 may also have a beam quality monitoring system 110 including a first CCD camera 112 and a second CCD camera 112.
FIG. 2 is an illustration of only the short axis optical assembly components of the assembly 20 of FIG. 1 and illustrates the utilization in the prior art Lambda Physik or JSW ELA machines of two laser beams 22, 22′ originating from two single chamber Lambda Phyik single oscillator excimer lasers, e.g., an XeCl laser or a KrF excimer laser. Also illustrated is the fact that the short axis imaging of the beam at the field stop 60 overlaps so that only part of the energy in the combined overlapped beam profiles passes through the slit 62.
FIG. 3 shows a possible lens combination 30 for both magnifying the beam in the short axis and expanding the beam in the long axis, which may comprise, e.g., a cylindrical convex lens 32 followed by a cylindrical concave lens resulting in a ling shaped beam 120 thinned in the axis of a scanning direction and elongated orthogonally to the scanning direction.
FIG. 4 shows an example of a lens assembly illustrative of the effect of beam homogenization carried out in the long axis beam homogenizer 40 and the short axis beam homogenizer 50 according to aspects of an embodiment of the present invention, including a substrate 140, which may be made of a suitable material capable of optically tolerating the DUV light at the required intensity levels, e.g., MgF2 or CaF2 with a thickness of, e.g., about 1.2 mm and on wither side arrays of cylindrical refractive, plano-convex cylindrical microlenses, with, e.g., a lens pitch in the array of, e.g., about 300 microns, resulting in a divergence angle of, e.g., about 1°, and a far-field flat-top intensity distribution. Such a lens assembly may be obtained, e.g., from Suss Microoptics sold under the designation CC-Q-300 1°, 5 mm×10 mm, High Power Beam Homogenizer.
A. Voutsas, et al., “Effect of process parameters on the structural characteristics of laterally grown, laser-annealed polycrystalline silicon films,” Jour. of Appld. Phys., Vol. 94, No. 12 (Dec. 15, 2003) (“Voutsas”), the disclosure of which is hereby incorporated by reference, notes the importance of the parameter of beam profile, particularly in the short axis of the beam in regard to lateral crystal growth. This is, as Votsas notes, primarily determined by the relationship between the critical size of the beam shaping pattern, i.e., approximately the width in the case of a large aspect ratio rectangle and the resolving power of the projection optics, i.e., approximately the numerical aperture of the projection lens. This relationship determines the edge “sharpness” of the beam profile, i.e., the spatial requirement for the intensity to rise from, e.g., about 10% to about 90% of full intensity. Efficient SLS processes can require an abrupt beam-edge profile to minimize the inefficient utilization of laser energy as explained in more detail in Voutsas. In practice, this profile can be dictated by the diffraction limit of the projection optics due to the compromise between numerical aperture and depth-of focus requirements. FIG. 4 shows a typical prior art spatial intensity distribution on a substrate in the short axis of the beam and the portion selected, e.g., by the optics discussed above to provide, e.g., an 8-9 μm wide beamlet generated from a 5:1 projection lens with numerical aperture of approximately 0.05. The intensity range corresponding to the SLS process window is also shown in Voutsas. For intensity values outside this range, partial melting of the film can occur with the subsequent formation of fine grain poly-Si material where the intensity is too low. The sharpness of the beam profile, therefore, can define the extent of this regime at each edge of the beam (de). Depending upon the width of the beamlet, also random nucleation may occur at the center of the beam, if the LGL under the irradiation conditions is smaller than about half of the beam width. Voutsas also notes that if dL is the lateral growth length and dc is the width of the central, nucleated region:w=2de+2dl+dc where w is the beam width. Voutsas also notes, that for optimum utilization of the beamlet, it is required that de→0 and dc→0. Also according to Voutsas, the nucleated, “center” region can be effectively eliminated, e.g., by essentially decreasing the beamlet width, while the beam edge region may be restricted but never practically eliminated. Voutsas also points out that in addition to the limitations of the projection optics, another source of beam-profile distortion is focusing. For a given numerical aperture for the projection optics, the depth-of-focus can be determined, defining, e.g., the degree of variation in the distance between the projection lens and the surface of the irradiated sample that results in essentially negligible changes in the imaging capability of the lens, i.e., its resolving power. For example, if, for a given projection lens, the location of the sample plane exceeds this limit in depth-of-focus, distortion in the imaged beam profile can occur, e.g., manifesting itself as “blurring;” i.e., the imaged beam cannot be fully resolved, with either underfocus and overfocus. Under such conditions according to Voutsas, the maximum beam intensity decreases and the edge becomes more diffuse, e.g., has a less steep or abrupt slope. In other words, de increases and the probability of nucleation at the center of the beamlet also increases. Voutsas also notes the variation of LGL as a function of the degree of defocusing in mm from the best focal plane, e.g., the lateral growth length being reduced with defocusing, due to the increased edge diffusion in the beam profile, which can render an increasing part of the beam ineffective for lateral growth. Also noted is edge length variation as a function of defocusing, wherein the decrease in lateral growth length can be accompanied by a concomitant increase in the edge length, at increased defocusing. Voutsas indicates that increasing the laser fluence can somewhat compensate for the defocusing losses on LGL, however, agglomeration can pose a limitation as to the extent of such compensation, indicating that distortions in the beam profile, e.g., due to defocusing may have to be accounted for in determining the optimal substrate pitch to maintain for lateral growth continuity over large areas.
Glass substrates for AMLCD manufacture using ELA are known to include, e.g., Corning® 1737 AMLCD glass substrates, as described in Corning's MIE 101 (August 2002), 1737 display grade glass substrates, as described in Corning's PEI 101 (December 2002), Eagle2000™ display grade glass substrates, as described in Corning's PIE 201 (December 2002) and “Glass substrates for AMLCD Applications: Properties and Implications,” TIP 101, February 2002) and “Support Designs for Reducing the Sag of Horizontally Supported Sheets,” TIP 303 (February 2003), the disclosures of which are hereby incorporated by reference.
FIG. 4 shows schematically a process flow for the preparation of the excimer laser annealed (“ELA”) poly-Si films according to the prior art. K. Lee, A Study on Laser Annealed Polycrystalline Silicon Thin Film Transistors (TFTs) with SiNx Gate Insulator, Chapter 5, Electrical and Structural Properties of ELA Poly-Si Filns, http://tftlcd. khu.ac.kr/research/poly-Si/chapter5.html As shown in FIG. 4 an XeCl excimer laser system (not shown) with a rectangular beam shape, in an annealing apparatus provided by, e.g., JSW Corporation can be used. A buffer layer 134 of Sio2 is deposited on the clean glass substrate 132 by APCVD. A 70 nm thick amorphous-Silicon:hydroxide (“a-Si:H”) film 136 is deposited by PECVD as a starting material for the ELA. The a-Si:H film is dehydrogenated by an excimer laser with 94% overlap scanning at 150 mJ/cm2 to form a layer of amorphous-silicon 138. Finally, the dehydrogenated a-Si layer 138 is crystallized by ELA with 94% overlap scanning at 300° C. to form a poly-silicon layer 138. The laser energy density can be varied 240˜330 mJ/cm2 to find an optimum laser intensity to obtain a high quality poly-Si film 138.
The JSW ELA system has relatively simple optics for crystallization and activation but produces a microstructure that is small grained and has a TFT mobility performance of only about 100 and a throughput of only 25 sheets per hour, a relatively high maintenance cost per sheet and process margins that are energy sensitive though focus insensitive. The JWS 2 Shot SLS system has relatively complex optics for crystallization, produces 2-3 μm grains and a mobility of between about 150 and 200, with a throughput of 35 sheets per hour. It has a maintenance cost per sheet that is about 32% cheaper than the JWS ELA, and has process margins that are energy insensitive but focus sensitive.
FIG. 5 shows a beam profile having, about a 9 μm beam linewidth.
It is also known that for such applications to utilize excimer lasers, e.g., inert gas halide lasers, with the laser-active material composed of an inert gas atom and a halide atom (Cl, I, F etc.). Also rare gas halide lasers, such as XeCl, XeF, KrF and ArF, have been demonstrated and utilized effectively in recrystallization of Si film as indicated in J. J. Ewing, et al., Phys. Rev. A12, 129 (1975) and M. Hoffman, et al., Appl. Phys. Lett. 9, 538 (1976), including common types of such lasers as listed in the reference including XeF. The reference also notes that to crystallize an a-Si:H, many excimer lasers have been used such as ArF (193 nm), KrF (248 nm), XeCl (308 nm), XeF (351 nm). Among them, XeCl excimer lasers have the advantages of good gas stability, high absorption coefficient for the a-Si:H film at near wavenumber of 308 nm. Therefore, many companies adopt the XeCl laser for the production because of stable operation and high absorption coefficient for a-Si:H (˜106 cm−1) at 308 nm. K. Lee, A Study on Laser Annealed Polycrystalline Silicon Thin Film Transistors (TFTs) with SiNx Gate Insulator, Chapter 4, Experimental Details, tftlcd.kyunghee.ac.kr/research/poly-Si/chapter4. html
C. Kim, et al., “Excimer-Laser Crystallized Poly-Si TFT's with Transparent Gate, IEEE Transactions on Electron Devices, Vol. 43, No. 4 (April 1996), p. 576-579 (“Kim”), discusses the utilization of XeF laser light irradiation on the glass substrate side of an amorphous silicon thin film on glass composite to form a poly-silicon transparent gate electrode in the amorphous silicon adjacent the glass substrate amorphous silicon interface. Also discussed in sht use of this technique for forming such transistors for driver monolithic active matrix-liquid crystal displays (“AM-LCDs”). Kim points out that top side annealing of the amorphous silicon is done with excimer lasers, including XeCl and others, citing K. Sera, et al., “High-performance TFT's fabricated by XeCl excimer laser annealing of hydrogenated amorphous-silicon film,” IEEE Transactions on Electron Devices, Vol. 36,Np. 12, (1989), pp. 2868-72; Y. Morita, et al., “UV pulsed laser annealing of Si- implanted silicon film and low-temperature super thin-film transistors,” Jpn. J. Appl. Phys., Vol. 28, No. 2 (1989) pp. L309-L311; K. Shimizu, et al., “On-Chip bottom gate polysilicon and amorphous silicon thin-film transistors using excimer laser annealed silicon nitride gate,” Jpn. J. Appl. Phys., Vol. 29, No. 10 (1990), pp. L1775-L1777; K. Shimizu, et al., “high-performance poly-si thin-film transistors with excimer laser annealed silicon nitride gate,” Jpn. J. Appl. Phys., Vol. 32, No. 1B (1993), pp. 452-57; M. Furuta, et al., “Bottom-gate poly-si thin film transistors using XeCl excimer laser annealing and ion doping techniques,” IEEE Trans. Electron Devices, Vol. 40, No. 14 (1993) pp. 1964-69; and Y. Sun, et al., “Excimer laser annealing process for polisilicon TFT AMLCD application,” Record of 1994 Int. Disp. Res. Conf. (1994), pp. 134-47, the disclosures of each of which is hereby incorporated by reference. However, Kim discloses and suggests the use of XeF only for through the substrate irradiation to form transparent bottom gates.
It is known to utilize image channels formed by rows of lenses with an intermediate image with each channel imaging a limited angular section with a superpositioning of the partial images performed by a spatial superposition in an image plane, requiring erect imaging and with only next neighbor superpositioning to avoid off-axis aberrations. mstnews 2/03, http://www.suss-microoptics. com/downloads/Publications/Miniaturization_of_Imaging_Systems. pdf.
The use of fly's eyes lenses for intensity redistribution of excimer laser beams is known, as discussed in Y. Ozaki, et al., “Cyllindrical fly's eye lens for intensity redistribution of an excimer laser beam, Applied Optics, Vol. 28, Issue 1 (January 1989) p. 106, and B. Crowther, et al., “A fly's eye condenser system for uniform illumination,” Proc. of SPIE, International Optical Design Conference 2002, Vol. 4832 (2002), pp. 4832-35.